1. Field of the Invention
The present invention relates to a digital-control oscillator circuit provided with an oscillator element and capacitor element, capable of changing the oscillation frequency by changing the capacitance of the load capacitor through a digital signal.
2. Description of the Related Art
An oscillator circuit employing an oscillator element such as a quartz oscillator or a ceramic oscillator provides an oscillation frequency of a high accuracy and a high stability. The oscillator element needs a load. A capacitive element is commonly employed for the load. A change in a capacitance of the capacitive element (hereinafter, referred to as a load capacitor) causes a change in an oscillation frequency of the oscillator circuit.
Oscillator circuits, even when produced in a settled manufacturing process, sometimes have a fluctuation in the oscillation frequency. When the fluctuation goes beyond a tolerance range, the oscillation frequency is adjusted by varying the capacitance of the load capacitor so as to have the frequency fall within a permissible range.
Japanese Patents Laid-open S57-132406 and H05-218738 (hereinafter referred to as D1 and D2, respectively) describe oscillator circuits capable of adjusting an oscillation frequency by changing the capacity of the load capacitor. D1 describes a technique to change the capacitance of the load capacitor depending on the characteristic of the piezoelectric oscillator, while D2 describes a technique to change the capacitance of the load capacitor depending on the temperature variation.
FIG. 1 shows a traditional digital-controlled oscillator circuit of the Colpitts type. The oscillator circuit has a parallel-resonance tank circuit made up of a crystal oscillator 1, a first capacitor circuit 2 and a second capacitor circuit 3 connected in parallel.
Crystal (quartz) oscillator 1 is connected between gate line 11 and the ground potential.
The oscillator circuit is further provided with a source-follower circuit 4 made up of an N-MOS transistor 9 with a source thereof connected with an emitter-grounded constant-current source 10.
First capacitor circuit 2 has two voltage-dividing capacitors 5 and 6 in series with one electrode of capacitor 5 connected to the gate of transistor 9, their common connection 13 connected to output 14 of source follower 4 and another electrode of capacitor 6 being grounded.
First capacitor circuit 2 acts as a main load capacitor. The capacitors 5 and 6 of first capacitor circuit 2 have capacitances C1 and C2, respectively.
The values of C1 and C2 are determined from both the desired oscillation frequency and the predetermined ratio of C1 to C2, wherein the ratio C1/C2 is determined so as to ensure a stable oscillation of the oscillator circuit.
Second capacitor circuit 3 has capacitor 7 of capacitance C3 and switch 8 for connecting capacitor 7 in parallel to or disconnecting it from the tank circuit.
The tank circuit is connected between gate line 11 and the ground potential.
The bias potential for the gate line 11 is supplied from the junction of two voltage-dividing resistors 15 and 16 connected in series between voltage source VDD and the ground potential.
FIG. 2 illustrates the constitution of the Colpitts oscillator circuit shown in FIG. 1: (a) the basic circuit; (b) the equivalent circuit.
Since the transistor amplifier is a source-follower, the basic circuit of the Colpitts oscillator circuit is represented in a drain-grounded configuration.
In FIG. 2(a), the reactance of crystal oscillator 1 is represented as L, and the dotted lines represent on/off of switch 8. In FIG. 2(b), X.sub.2 denotes the equivalent reactance to the parallel reactances of crystal oscillator 1 and capacitor 7. It follows that X.sub.2 equals jL.omega. if switch 8 is off and equals -j[C.sub.3.omega.-(L.omega.).sup.-1 ].sup.-1 if switch 8 is on. In the figure, v.sub.A and v.sub.B denote an input voltage (gate-source voltage) and an output voltage (source-ground voltage), respectively. The g.sub.m and g.sub.d stand for a transconductance and an internal conductance, respectively, and g.sub.m v.sub.A represents a voltage-controlled current source, as usual.
The equivalent circuit gives an admittance y.sub.0 =i.sub.o /v.sub.B as viewed from 1 and 1' as follows: ##EQU1##
where ##EQU2##
X.sub.2 =jL.omega. when switch 8 is off, and ##EQU3## PA1 when switch 8 is on.
Equating Imaginary(y.sub.0) to 0 gives the frequency condition EQU X.sub.2.omega.[C.sub.1 C.sub.2 /(C.sub.1 +C.sub.2)]=1. (4)
Generation of a stable oscillation requires further condition for sustaining a resonance oscillation without decay. This condition will hereinafter be referred to as an oscillation condition.
The oscillation condition is given by the inequality EQU Real(y.sub.0)&lt;0, (5)
i.e., ##EQU4##
Substitution of equation (4) to equation (6) yields the oscillation condition for a resonance frequency: ##EQU5##
It is to be noted that the oscillation condition (5) is independent of X.sub.2 at the resonance frequency as represented in inequality (7), while it relates to X.sub.2 as represented in inequality (6) when the frequency is not at a resonance frequency.
It is an object of the present invention to provide a digital-control Colpitts crystal oscillator circuit of a modified type capable of changing an oscillator frequency sustaining a stable oscillation.